A rare inherited blood disorder where the hemoglobin (the oxygen-carrying part of red blood cells) can’t bind with oxygen which impairs the oxygen supply to parts of the body, especially during exertion when more oxygen is needed by the body. Type I has enzyme deficiency (cytochrome B5 reductase) only in the red blood cells whereas in Type II the enzyme deficiency occurs in other body cells as well as red blood cells. See also Hereditary methemoglobinemia, recessive